Spirobolida
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The order Spirobolida is a species-rich (more than 900 described species) and important group of millipedes. Most spirobolids live in the tropics, and like the majority of millipedes, members of the order are all detritivores. Species are often large and conspicuous, and at least in the United States, are often used as classroom representatives for the class Diplopoda. Taxonomically, it is a relatively neglected group with only two revised families, and both revisions dating back more than 40 years.
Spirobolida is one of 11 helminthomorph diplopod orders. Helminthomorphs are typified by the modification of at least one pair of legs on the seventh segment of males into secondary sperm-transfer organs called gonopods. Gonopod morphology is usually species specific and of great taxonomic importance. Much of helminthomorph taxonomy is based almost entirely on gonopod structure (especially at lower taxonomic levels). Female sexual organs (cyphopods) are relatively understudied and underused in millipede taxonomy, with the unfortunate result that female specimens are often unidentifiable to species level.
Spirobolids have both pairs on legs of the seventh segment modified into gonopods, which are thus divided into an anterior pair, the coleopods, and a posterior pair, the phallopods. It is the phallopods that assume the most active role during sperm transfer. The coleopods seem to have a mostly protective function and usually envelope the phallopods. It is the phallopods that display the highest level of species specificity. Development in these animals is gradual and gonopods are first formed during several nymphal stages and molts preceding the final adult form. Growth, and therefore moulting, continues through adulthood, and their life span is typically several years.
